1. Technical Field
The present disclosure relates to a method for manufacturing a sensor device of a gaseous substance of interest.
The disclosure also relates to a sensor device thus realized and to a sensing system comprising it.
The disclosure relates, even more in particular, but not exclusively, to a method for manufacturing a sensor device of a gaseous substance of interest of the type comprising at least one thin film transistor comprising a layer sensitive to gaseous substances, in particular toxic, such as for example nitrogen dioxide (NO2), and the following description is made with reference to this field of application by way of illustration only.
2. Description of the Related Art
As it is well known, several inflammable, explosive, toxic gases are normally present in inhabited environments and, then, studying and realizing gas sensors for the sensing of these dangerous gases becomes of fundamental importance for the aim of reducing the environmental contamination and preserving human health.
Among gas sensors those comprising substances of organic nature or metallic-organic compounds able to detect the presence of one or more substances present in the environment in the gaseous form are well known, such as for example sensors of carbon monoxide (CO) or of oxygen (O2). Several substances of organic nature and metallic-organic compounds are in fact known able to detect the presence or not of one or more substances present in the environment in the gaseous form through physical and chemical mechanisms of different nature and, then, able to perform, in general, sensing functions towards the same species of interest.
Among the various gas sensors, particular attention has been paid to semiconductor gas sensors by virtue of their low cost, of the small volumes, of the simple realization and of the high sensitivity. In particular, different oxides have been used as semiconductor materials for these semiconductor gas sensors, such as for example tin oxide (SnO2), iron oxide (Fe2O3), manganese dioxide (MnO2) and chromium monoxide (CrO).
Intense research and development activities are in particular aimed at the identification of suitable semiconductor materials and at the integration of the same in low cost electronic devices for the sensing of specific substances with a high selectivity and sensitivity degree towards the species of interest, in particular of those substances, considered toxic for humans beyond determined exposure levels, whose continuous monitoring plays, as said, a role of fundamental importance in all the environments frequented by humans, either domestic, or commercial, or public or industrial.
In this case, most of the materials having sensing functions towards this gaseous substance of interest comprise hybrid materials with a metallic component, in particular metallic oxides and/or metallic-organic compounds, based for example on Zinc (Zn), Indium (In), tin (Sn), Tungsten (W) and Molybdenum (Mo).
As it is well known, in fact, in the process of gas sensing executed by these sensors, the gas is adsorbed on the surface of the semiconductor material; in this way, according to the fact that the electronic affinity of the molecular gas is greater of smaller than the working function of the semiconductor material used, there is a transfer of electrons from the surface of the semiconductor material to the molecular gas or vice versa. This movement of electrons determines a variation of the electric resistance of the semiconductor material itself that, thus, may be related to the concentration and to the type of molecular gas detected.
It is also known that it is desirable for the gas sensing material to have a high sensitivity, a strong selectivity, a good stability, short response and recovery time. A semiconductor material having such characteristics is for example zinc oxide (ZnO), that has been, in fact, used in different known solutions of gas sensors. Zinc oxide (ZnO) is a semiconductor of the n type belonging to the family of the groups II-VI, having direct <<gap>> and distance between valence and conduction bands (Eg) equal to 3.37 eV. This characteristic, together with the high excitonic bond energy, i.e., the energy for separating electron and hole linked by colombian interaction, in the case of zinc oxide (ZnO) equal to about 60 meV, makes it suitable for electronic and/or optoelectronic applications, but also for applications within the sensorial domain.
In fact, zinc oxide (ZnO) has, besides the excellent electric characteristics, i.e., high mobility and wide band gap, low cost, non-toxicity, good environmental stability, high melting point and high transparency against the visible spectrum, with subsequent stability to the visible light with respect to other semiconductors with low band gap.
Gas sensors may be for example of the type with thick film or with thin film.
Gas sensors of the “thick film” type may be realized, for example, by means of the so called “screen printing” technology. In particular, according to this technology, a measuring electrode, a heating electrode and a sensitive paste containing a semiconductor material, a catalyst and an adhesive, are first printed on a substrate of the high temperature type, for example ceramic, and then sintered.
For gas sensors of the “thin film” type, a sensitive thin film is first spread on a ceramic substrate by means of vacuum thin film technology and, subsequently, on this thin film an electrode is formed.
All the gas sensors above described are of the type with two electrodes. They comprise in particular a positive electrode and a negative electrode, and a semiconductor, interposed between the two electrodes, for detecting the variation of electric resistance, and, then, of the electric current that flows between the two electrodes, according to the type and to the concentration of gas adsorbed on its surface.
At present, however, sensors with two electrodes are realized on little economic substrates that should sustain high temperature processes. Moreover, the measurement is based on the change of the electric resistance of a passive element with two terminals (such as a resistor) realized therein and, therefore, this measurement is not accurate enough in the cases of low concentrations of toxic gas to be detected.
Recent studies have led to solutions with gas sensors having three electrodes, wherein a gate electrode is added to the two electrodes, here with function of source and drain electrodes, realizing de facto a sensing transistor and optimizing the performances of the sensor itself, with particular reference to the sensitivity of the sensor for small amounts of substance to be detected.
An example of such a sensor is described in the US patent application with publication number US 2010/0050745, filed on Sep. 3, 2008 by National Formosa University. In this application a gas sensor is described that is realized with a Field Effect Transistor or FET based on nanowires of zinc oxide (ZnO) comprising a channel of charge carriers made of nanowires of zinc oxide (ZnO) comprised between the source and the drain of the transistor. The charge flow through the channel, and, thus, the electric resistance of the nanowires of zinc oxide (ZnO) is controlled by the gate terminal of the FET. Substantially, then, a small variation of the gate voltage may affect the electric current between source and drain.
Although advantageous under several aspects, this solution has several drawbacks. In fact, the realization process is complicated by the fact that the nanowires of zinc oxide (ZnO) are deposited before the realization of the source and drain electrodes, involving high temperature process steps, non-compatible with the cheaper and more flexible typologies of substrates such as for example plastic substrates. Moreover, the FET described in this solution comprises a metallic layer suitable for realizing a heating element. Finally, the FET described is not specifically designed for the sensing in the air of toxic gaseous substances, such as for example nitrogen dioxide (NO2).
In particular, it is known that the presence of nitrogen dioxide (NO2) is tolerated in amounts not larger than 3 ppm for an exposure time not higher than 8 hours, or in amounts not larger than 5 ppm for an exposure time not higher than 15 min.
Thus, it becomes of fundamental importance, in the microelectronics industry, to favor the production and the marketing of systems, such as exactly the gas sensors, for the sensing and the monitoring of gaseous substances, in particular dangerous gases, to significantly reduce the costs of the single elements constituting them and to increase their reliability, the sensitivity, the specificity, the stability and the mechanical strength.
More in particular, the need is felt of realizing low cost sensors of nitrogen dioxide (NO2) able to detect minimal amounts of this gas, for example of the order of ˜1 ppm.